Thailand's Department of Special Investigation (DSI) has denied that it was pressured to clear the army of responsibility for the death of a Japanese cameraman during protests last year.

An earlier report found that Reuters cameraman Hiro Muramoto was probably killed by a soldier's bullet as he was filming clashes between the army and anti-government demonstrators last April.

However a new report says that he was hit with a bullet from an AK47, which is not used by the Thai military.

Investigators deny that they were leant on to change their finding after the head of the army expressed displeasure.

DSI chief Tharit Pengdit told reporters on Monday that neither the government nor the military had interfered in the probe.

"There's no pressure," he told reporters. "I can confirm that we are doing our jobs as normal."

He said that the initial conclusion that Muramoto might have been killed by the military was based on a witness account from a policeman, who said the shot had come from an area where there were both soldiers and protesters.

Reuters called for clarification.

"The apparent contradiction between the preliminary investigation and these reports makes full transparency about the process and the findings imperative," Stephen Adler, editor-in-chief of Reuters News, said in a statement.

Muramoto, 43, was one of two foreign cameramen killed during the unrest in April and May. More than 90 people - mostly civilians - died in the clashes between troops and protesters.

Both sides accused each other of using live ammunition.

A police forensic expert, Amporn Charuchinda, told reporters the autopsy report and pictures suggested that Muramoto was killed by a bullet with a diameter of at least seven millimetres, which could have come from an AK-47.

The Japanese embassy declined to comment on the new conclusion.

Meanwhile, Thailand's opposition has filed an impeachment motion against the prime minister and others ahead of a no confidence motion due to the use of force in last year's protests, which began a year ago next week.